Imbalances between stem cell self-renewal versus differentiation, as well as malfunctioning of stem cell derivatives are common causes of many human diseases, including infertility and cancer. The Polycomb group (PcG) transcription repressive proteins are key regulators of both embryonic stem cells and adult stem cells. Tight control of PcG activity is critical for maintaining the balance between stem cell self-renewal/proliferation and differentiation. Down-regulation of PcG is a prerequisite for stem cells to stop self-renewal/proliferation and switch to differentiation. Consequently, enhanced PcG activities in stem/precursor cells result in cancers, such as leukemia. However, little is known about how normal expression and activity of PcG genes are regulated and how misregulation leads to disease. It is our long-term goal to apply our research results for therapeutic design to prevent infertility, cancer, and many other human diseases. The Drosophila male germline stem cell (GSC) provides an excellent in vivo model system to study stem cell identity and activity. Using this stem cell lineage, we have previously shown that the PcG proteins repress expression of terminal differentiation genes in undifferentiated cells. An orchestrated developmental program reverses the PcG silencing and turns on differentiation genes. This work uncovered an intriguing parallel between the GSC lineage and mammalian stem cell lineages. The goal of this proposal is to use this well-established stem cell system to investigate how PcG activities are tightly controlled during germ cell differentiation and how PcG proteins regulate GSC maintenance, proliferation, and differentiation. Results from our studies will have a broad impact on reproductive biology, stem cell biology, and regenerative medicine.